back when I worked at the LFS, we had many customers try it when it first came out (granted, it is probably an updated formula now) and all had problems.
they stopped water changing their fish, starting using the stuff (since it used to say on the bottle you don't have to water change your tank for up to 6 months

when using it..), and most came in with dead fish/bad water all the time.
we never pushed it at the store. it was available, but we never recommended it.
still, customers would try it anyway. this was back in the early 2000's.
I have never used it myself.
as far as higher nitrates in tap water to begin with, if that is a problem in your area, your best bet for that is to use a "nitrazorb" type filter media in your canister/HOB, recharge perodically and go from there.
sometimes in my area we have people on well water where the nitrates were 20mg/L to begin with, this method with the nitrazorb (a seachem product if I recall) worked well for them. also make sure to use a good water conditioner as well water can sometimes have trace amounts of heavy metals in it that a good water conditioner will render harmless to the fish.
my two cents.